Chiral Separation of Pesticides Using CHIRALPAK IG Under Polar Organic Mode and Reversed-Phase High-Performance Liquid Chromatography. Influence of Mobile Phase Composition and Temperature on Enantioselectivity

ECHEVARRIA, ROMINA N.; DIAZ MERINO, MATIAS EZEQUIEL; LUBOMIRSKY, ESTER; PADRO JUAN MANUEL; CASTELLS, CECILIA B.

Washington, DC, USA

Simposio; 47th International Symposium on High Performance Liquid Phase Separations and Related Techniques (HPLC2018); 2018

Polysaccharide-type CSPs are most frequently used under normal phase conditions although enantiomer resolution by aqueous eluent has been also used [1?3]. The choice of reversed-phase mode (RP) was often related to analyte scarce retentivity, low sample solubility of polar species and, less frequently, lack of enantioselectivity in organic eluents. In the last decades, an additional reason to prefer RP methods has been their suitability for LC?MS applications [1]. If sample components are soluble in aqueous medium and an HPLC separation on polysaccharide-derived column will be tested, there are two possibilities: (a) to develop a method in polar organic mode, or, (b) to develop the method under reversed-phase conditions.The effect of temperature and mobile phase composition are the key for achieving resolution of specific solutes in a given the stationary phase. This is especially true for enantiomeric separation. These operating variables affects retention, selectivity, system pressure, and column stability. Moreover, by varying column temperature, thermodynamic tests may be used to intend to reveal the underlying mechanism of chiral recognition.In this work, a new recently introduced amylose tris(3-chloro-5-methylphenyl-carbamate), commercialized as CHIRALPAK IG ®, was used under polar organic mode and reversed-phase conditions for the chiral separation of 30 pesticides widely used as herbicides, fungicides, insecticides, anticoagulant rodenticides, and precursors. The mobile phases were methanol, ethanol, iso-propanol, n-propanol or acetonitrile and their mixtures with water, with or without added additives such as diethylamine, trifluoroacetic acid, formic acid or acetic acid or mixtures thereof, with compositions comprised between 0.1 to 0.4%. The influence of mobile phase composition and column temperature (from 0 to 40 ºC) on the separation was investigated.We will discuss the different behavior in retention factors (k), selectivity factor (α) and asymmetries for the enantiomers of all theinvestigated pesticides. Practically all those compounds decreased their k and α with the increase in temperature. The ln α vs. 1/T and ln k vs. 1/T plots for enantiomers of chiral pesticides were linear at the range of 0?40 ºC when we used polar organic mode, but the effect of changing the nature of the mobile phase components was extremely difficult of prediction. Estimation of the thermodynamic parameters based on linear van?t Hoff plots are also discussed. [1]Zhang, T., Nguyen, D., Franco, P., J. Chromatogr. A 2010, 1217, 1048?1055.[2]Padró, J. M., Keunchkarian, S., Microchem. J. 2018, 140, 142?157.[3]Tachibana, K., Ohnishi, A., J. Chromatogr. A 2001, 906, 127?154.